Products pursuing high quality and high luminance, such as TV or signage in particular, have been employing liquid crystal displays having the display contrast enhanced by local dimming. Local dimming here is a technology of partially controlling light sources disposed in a backlight device to control the amount of light from individual light sources. Particularly, so-called direct backlight devices, including light sources disposed at the back surface of a light-emitting surface of an illumination device included in the backlight devices, have been increasingly used since the direct backlight devices facilitate driving of local dimming.
Direct backlight devices have been widespread among displays including car-mount small displays these years. Direct backlight devices, however, have the following problems for use under predetermined high-temperature environments.
In direct backlight devices in an oversize display, a reflection sheet is usually partially fixed to a light-source substrate with rivets or other components. However, in direct backlight devices in small and midsize displays, a reflection sheet is merely placed on the light-source substrate without being fixed to the light-source substrate with rivets or other components, unlike in the case of an oversize display. Moreover, the reflection sheet is located lower than the upper surfaces of the light sources for enhancing the light use efficiency.
With reference to FIG. 17 to FIG. 19, a structure of an illumination device 2 included in a typical direct backlight device will be described. As illustrated in FIG. 17, the illumination device 2 includes a light-source substrate 1, including multiple light sources 7 such as LEDs arranged side by side, and a reflection sheet 9, which is a single sheet over a light-source substrate 20 mainly between the light sources 7. The reflection sheet 9 has multiple light source openings 3, which individually open the multiple light sources 7. Here, to open represents to let the light sources 7 exposed without being covered by the reflection sheet 9 disposed over the light-source substrate 1.
The illumination device 2 includes a diffusion sheet 6, disposed to face light-emitting surfaces 7a of the light sources 7 of the light-source substrate 1. The light-source substrate 1 has its surface coated with a white resist 1a. As in the reflection sheet 9, the white resist 1a has a white reflection surface having high light reflectivity to enhance the light use efficiency of the illumination device 2.
Specifically, the white resist 1a is disposed on a portion through which the light-source substrate 1 on which the reflection sheet 9 is disposed is exposed.
Thus, most part of the portion of the light-source substrate 1 on which the light source 7 is not disposed is covered with the white reflection sheet 9 and the white resist 1a having high light reflectivity.
A diffusion sheet 6 has a function of diffusing light from the light sources 7, the white resist 1a, and the reflection sheet 9.
The reflection sheet 9 is subjected to stretching processing in a predetermined direction during the manufacturing step. Thus, the reflection sheet 9 thermally contracts in the extension direction when heated. A typically mass-produced reflection sheet 9 has a small thickness such as 0.188 mm or 0.225 mm, and thus is more likely to thermally contract. Thus, when the above-described illumination device is used under the high-temperature environment, the reflection sheet 9 contracts in the extension direction with the thermal effect, and may overlap the light source 7. The light source 7 covered with the reflection sheet 9 locally causes bright and dark portions and uneven luminance, and degrades the display quality.
To address this, PTL 1 describes an illumination device that has slits around the openings of the reflection sheet. The slits can effectively solve thermal distortion of the reflection sheet. Compared to the structure including, for example, openings, this illumination device has a wider reflection surface, and is thus assumed to prevent reduction of light use efficiency.